True, but most definitely IS necessary for the films to make sense as a quintet. Galadriel is not necessary for the Hobbit, but has been cast. Drogo has been cast, not in The Hobbit. We never see or hear Radagast in The Hobbit, but a well known actor has been cast. We know these things. It's clear that we are going to be seeing more than just the Hobbit story, and, imo, the reason for that is to tell the whole story over 5 films. I cannot see that these two new films will only tell the Hobbit story. In fact, It's obvious that they won't by the castings.

Jackson et al made the first three films for people who have, and have not read the books. Balin going to Moria may not need to be explained for the Hobbit story per se, and not for readers of Tolkien's works. For the movies though, that has to be explained. It makes no sense otherwise.

It's pretty clear to me why the bridge film concept was abandoned, but it was abandoned as a film. I think some of that material will still be included, and the castings support that. Mawr yw ein braint i berthyn i'r gwm Llynfi